Pulsed laser systems, such as excimer lasers, are well known. FIG. 1 is a front view of laser chamber 10 used in a conventional pulsed laser system. As shown in FIG. 1, laser chamber 10 includes an electrode structure 12, a blower 14, a heat exchanger 26, a pre-ionizer 28, baffles 30 and a grounding screen 32, which is used to connect the lower of electrodes 12 to ground. A laser discharge area 24 is defined by electrode structure 12.
FIG. 2 is a side view of laser chamber 10. Laser chamber 10, as shown in FIG. 2, also includes windows 16, 18, and a laser beam 20.
As well known by those skilled in the art, a pulsed laser system, such as an excimer laser, produces high energy, high frequency pulses in a gas that is between electrodes 12 in laser chamber 10. The gas, which may contain krypton and fluorine, is maintained at high pressure, for example, 3 atm. Pre-ionizer 28 first floods the gas within discharge area 24 with free electrons (10.sup.6 to 10.sup.8 per cm.sup.3). Once the gas within discharge area 24 is conditioned with a sufficiently increased electron density, electrodes 12 produce a high energy discharge, which may be for example 15-50 kV. The lasing action from the high energy discharge occurs within 100 nsec from the time of discharge.
The high energy discharge in discharge area 24 produces a large amount of local heating in the gas. The thermal disturbance changes the index of refraction of the gas, which has a deleterious effect on the efficiency and energy of the laser system. The thermal disturbance in the gas does not affect the lasing action from the pulse that caused the thermal disturbance because the lasing action occurs within a short amount of time after the high energy discharge, approximately 100 nsec. However, subsequent high energy discharges, which occur at a frequency of approximately 1 to 2 KHz, will be produced in the highly disturbed, thermally energetic gas unless the gas is circulated within laser chamber 10. Thus, blower 14 is used to circulate the gas within laser chamber 10, while heat exchanger 26 is placed in the path of the gas flow to cool the gas as it circulates. Typically, the gas in laser chamber 10 is circulated with a flow velocity of 25-30 meters per second, however this amount is dictated by the frequency of the pulsed laser system.
When electrode structure 12 produces a high energy discharge an acoustic and/or shock wave is produced which then propagates outward from discharge area 24. An acoustic wave is a standing wave that is formed within the cavity of laser chamber 10 and that travels with the velocity of sound. Shock waves, on the other hand, are free standing waves that generate high pressure gradients and that travel at velocities greater than the velocity of sound.
The acoustic and shock waves propagate through the gas until they reach the walls 11 and 13 of laser chamber 10, at which time the acoustic and shock waves are reflected back into discharge area 24. The acoustic and shock waves are unwanted pressure changes in the gas that, when reflected back into discharge area 24, interfere with the energy efficiency and energy stability of the laser system. The degree to which the energy efficiency and energy stability are affected depends on pulse repetition frequency as these frequencies interact with the natural acoustic modes of the chamber.